The Elder Scrolls V: Skyrim

It has been happening more often that I kill a dragon and it has no soul. thats a bug that needs to be fix. I have 4 small mods that change minors things. some times it has a soul, sometime it does not.

It sounds like this probably isn't the problem you're having, but just in case: if you have Dragonborn and the Unofficial Skyrim Patch but not the Unofficial Dragonborn Patch as well, it seems to often cause this problem.

Once you complete the quest The Temple of Miraak, Miraak will begin stealing dragon souls from you, whether you are in Skyrim or on Solstheim. The first time you kill a dragon, he will always appear, and thereafter every time you kill a dragon with a soul that can be absorbed, there is a 25% chance that Miraak will appear. When this occurs, he will be in a semi-transparent and invulnerable state, and will absorb the soul of the slain dragon, preventing you from obtaining it. He will then disappear. This event will continue to happen until Miraak is defeated during the quest At the Summit of Apocrypha.